The present invention relates to a duplex stainless steel having excellent corrosion resistance in a chloride-containing solution as well as improved toughness and workability, and a process for the production thereof.
Recent developments in the gas atomization process to prepare stainless steel powder and the powder compaction process to produce stainless steel products make it possible to produce those stainless steels which are difficult to manufacture by a conventional melting method which includes melting, casting and forging.
Duplex stainless steels are known to have high strength and excellent resistance to pitting corrosion, crevice corrosion, and stress-corrosion cracking, and they are nevertheless less expensive than austenitic stainless steels. Typical commercially-available duplex stainless steels contain 18 -26% Cr, 4 -8% Ni, and 1 -3% Mo. As the field of applications of duplex stainless steels is expanded, further improvements in their properties have been desired.
For example, it is possible to further improve the corrosion resistance of a duplex stainless steel by increasing the Cr and Mo contents thereof. However, when it is prepared by a conventional melting process, the formation of intermetallic compounds occurs inevitably, thereby causing a decrease in toughness of the steel.
Japanese Patent Applications Laid-Open Nos. 61-243149(1986) and 62-222043(1987) disclose the production of high-Cr, high-Mo duplex stainless steels by the powder metallurgy method, i.e., a combination of the above-described gas atomization and powder compaction processes, which eliminates embrittlement of the stainless steel products caused by precipitation of intermetallic compounds. The precipitation of intermetallic compounds during preparation of such stainless steels was thought to be unavoidable in a conventional melting process. In contrast, application of the powder metallurgy method makes it possible to realize an increase in the Cr and Mo contents of a duplex stainless steel, which is desired for such a steel, without precipitation of intermetallic compounds.
Japanese Patent Application Laid-Open No. 62-56556(1987) describes the preparation by the melting method of a high-Cr, high-Mo duplex stainless steel containing 23% -27% Cr and 3.5%-4.9% Mo by weight. However, the Cr content of suc virtually limited to 25% by weight or less in order to prevent the formation of chromium nitride and intermetallic compounds. Therefore, it is not ensured that the steel has fully improved corrosion resistance.
The production of stainless steel powder by the gas atomization process is normally conducted either (1) by merely remelting a previously-prepared master alloy in an inductionheating furnace to form a molten alloy, which is then forced through a small orifice by a rapid stream of an inert gas for atomization (remelting method), or (2) by melting individual alloying metals together in a similar furnace in which the proportions of the alloying metals are adjusted so as to form a molten alloy having the desired alloy composition, followed by atomization in the above manner (melting method).
In the case of a high-Cr, high-Mo duplex stainless steel, it is difficult to previously prepare a master alloy for remelting since it is brittle and difficult to work by forging or other means into a prescribed shape of a master alloy. Therefore, the above-described method (2) is solely employed in the preparation of a powder of such a duplex stainless steel.
According to this method, however, refining treatment such as desulfurization or deoxidation can normally not be performed on the resulting molten alloy during melting in an inductionheating furnace. Therefore, particularly in the preparation of a high-Cr, high-Mo stainless steel powder, this method tends to give a steel powder product having an increased oxygen content due to a high susceptibility of chromium to oxidation. As a result, the resulting powder has a decreased hot workability and therefore it is difficult to compact into a desired shape by means of hot working. In addition, the amount of inclusions formed in the resulting steel is so increased that the cleanness and hence the corrosion resistance of the steel are degraded. In order to produce a steel powder having a decreased oxygen content, it is necessary not only to control the surrounding atmosphere but also to use pure alloying metals as raw materials. However, unlike a laboratory experiment, it is difficult for industrial-scale production of stainless steel powders to meet such conditions.
Furthermore, although the use of the powder metallurgy method in the production of a high-Cr, high-Mo duplex stainless steel can produce a compacted body without embrittlement due to precipitation of intermetallic compounds, the subsequent cooling of the compacted body is accompanied by precipitation of intermetallic compounds. Therefore, in this method as well, the product is brittle and is difficult to transport and subject to cold working and machining.